Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Some of the components in the power flow path between the engine and the wheels have some degree of torsional compliance. The components also have torsional moments of inertia. As a result, various oscillation modes are possible. In some cases, it is possible to adjust the compliance and inertia to ensure that the natural frequency is not excited by typical operations. In other cases, energy absorbing components, such as an open torque converter, prevent unpleasant oscillations. However, operating with energy absorbing components increases parasitic drag and reduces improve fuel economy